The present invention, generally, relates to the field of mechanical drive systems, and, more particularly, to systems which produce linear motion under servomechanism control.
A variety of electromechanical systems use linear actuators to move a load under control. An important application of such systems is for simulator motion bases, in which a simulator cockpit or cab is moved in accordance with motion models of the vehicle being simulated.
There are two traditional methods for providing controlled linear actuation in simulator motion bases and other such systems. One traditional method is to use an electric motor to power a hydraulic pump, then control the flow of hydraulic fluid to a hydraulic piston using an electrically controlled servo valve.
A second method is to use a controlled electric servomotor to drive a ball screw mechanism. Variations of the second method are to use an electric servomotor to drive a gearbox or a system of belts and pulleys and, ultimately, a bell crank that provides a linear motion.
A totally hydraulic system, the first method, provides smooth controllable power, but such a system is complex and has limited efficiency. The hydraulic pump works continuously to maintain the system pressure. Fluid is pumped through a valve set at the operating pressure of the system, and if no motion is developed, the oil is returned to a reservoir at atmospheric pressure. This heats the oil.
In addition, the precision servomechanism valves, required to control the flow to actuating cylinders, require the control system and amplifiers of an electric servo system and the narrow opening of the valve produces a pressure drop with further inefficiency and oil heating. The fluid must be carefully filtered so as to prevent damage to the precision servo control valves.
Pure electric systems, those of the second method, are limited because electric motors tend to produce high torque only at high rotational speeds. Converting high speed rotary motion to low speed linear motion necessitates ball screw, gear box, or pulley-and-bellcrank arrangements.
Ball screws are expensive, tend to provide rough motion, and are prone to wear. Gearbox arrangements are expensive also and are prone to rough motion due to backlash when reversing direction. Pulley arrangements are constrained by the size and the mounting requirements of the pulleys and, therefore, are impractical to fit into many designs.
Other drive mechanisms have been used occasionally, such as linear motors. These tend to be expensive and may have mechanical constraints unsuited to particular applications.
Closed-circuit hydraulic systems, which use hydraulics without the necessity of a reservoir and constant-pressure pump, are known in prior art for achieving coordinated motion. Hydraulic braking systems couple the motion of one cylinder to another, for example.
U.S. Pat. No. 5,018,950 to Reinhart describes a system using a linear electric motor that actuates a hydraulic piston which, in turn, actuates a second hydraulic piston.